Lamivudine (3TC), the negative enantiomer of 2'-deoxy-3'-thiacytidine, is a dideoxynucleoside analogue used in combination with other agents in the treatment of human immunodeficiency virus type 1 (HIV-1) infection and as monotherapy in the treatment of hepatitis B virus (HBV) infection. Lamivudine undergoes anabolic phosphorylation by intracellular kinases to form lamivudine 5'-triphosphate, the active anabolite which prevents HIV-1 and HBV replication by competitively inhibiting viral reverse transcriptase and terminating proviral DNA chain extension. The pharmacokinetics of lamivudine are similar in patients with HIV-1 or HBV infection, and healthy volunteers. The drug is rapidly absorbed after oral administration, with maximum serum concentrations usually attained 0.5 to 1.5 hours after the dose. The absolute bioavailability is approximately 82 and 68% in adults and children, respectively. Lamivudine systemic exposure, as measured by the area under the serum drug concentration-time curve (AUC), is not altered when it is administered with food. Lamivudine is widely distributed into total body fluid, the mean apparent volume of distribution (Vd) being approximately 1.3 L/kg following intravenous administration. In pregnant women, lamivudine concentrations in maternal serum, amniotic fluid, umbilical cord and neonatal serum are comparable, indicating that the drug diffuses freely across the placenta. In postpartum women lamivudine is secreted into breast milk. The concentration of lamivudine in cerebrospinal fluid (CSF) is low to modest, being 4 to 8% of serum concentrations in adults and 9 to 17% of serum concentrations in children measured at 2 to 4 hours after the dose. In patients with normal renal function, about 5% of the parent compound is metabolised to the trans-sulphoxide metabolite, which is pharmacologically inactive. In patients with renal impairment, the amount of trans-sulphoxide metabolite recovered in the urine increases, presumably as a function of the decreased lamivudine elimination. As approximately 70% of an oral dose is eliminated renally as unchanged drug, the dose needs to be reduced in patients with renal insufficiency. Hepatic impairment does not affect the pharmacokinetics of lamivudine. Systemic clearance following single intravenous doses averages 20 to 25 L/h (approximately 0.3 L/h/kg). The dominant elimination half-life of lamivudine is approximately 5 to 7 hours, and the in vitro intracellular half-life of its active 5'-triphosphate anabolite is 10.5 to 15.5 hours and 17 to 19 hours in HIV-1 and HBV cell lines, respectively. Drug interaction studies have shown that trimethoprim increases the AUC and decreases the renal clearance of lamivudine, although lamivudine does not affect the disposition of trimethoprim. Other studies have demonstrated no significant interaction between lamivudine and zidovudine or between lamivudine and interferon-alpha-2b. There is limited potential for drug-drug interactions with compounds that are metabolised and/or highly protein bound.

. Studies of viral genotype and phenotype after virus rebound revealed that the initial active site mutation allowing for nelfinavir resistance is mediated by a unique amino acid substitution in the HIV-1 protease D30N, which does not confer in vitro phenotypic cross-resistance to the currently available protease inhibitors.

Coadministration of lamivudine with trimethoprim-sulfamethoxazole resulted in an increased AUC infinity and a decreased CLR of lamivudine. However, given the favorable safety profile of lamivudine, it is unlikely that this interaction will result in a significant increase in concentration-related toxicity at the doses studied.

Abacavir is a carbocyclic 2'-deoxyguanosine nucleoside reverse transcriptase inhibitor that is used as either a 600-mg once-daily or 300-mg twice-daily regimen exclusively in the treatment of HIV infection. Abacavir is rapidly absorbed after oral administration, with peak concentrations occurring 0.63-1 hour after dosing. The absolute bioavailability of abacavir is approximately 83%. Abacavir pharmacokinetics are linear and dose-proportional over the range of 300-1200 mg/day. To date, one study has assessed the steady-state pharmacokinetics of abacavir following a 600-mg once-daily regimen, and reported a geometric mean steady-state abacavir peak concentration of 3.85 microg/mL. Although this concentration is higher than the steady-state abacavir peak concentration reported following a 300-mg twice-daily regimen (0.88-3.19 microg/mL, depending on the study), the geometric mean steady-state abacavir exposure over 24 hours was similar following these regimens. Coadministration with food has no significant effect on abacavir exposure; therefore, abacavir may be administered with or without food.The apparent volume of distribution of abacavir after intravenous administration is approximately 0.86 +/- 0.15 L/kg, suggesting that abacavir is distributed to extravascular spaces. Binding to plasma proteins is about 50% and is independent of the plasma abacavir concentration. Abacavir is extensively metabolized by the liver; less than 2% is excreted as unchanged drug in the urine. Abacavir is primarily metabolized via two pathways, uridine diphosphate glucuronyltransferase and alcohol dehydrogenase, resulting in the inactive glucuronide metabolite (361W94, ~36% of the dose recovered in the urine) and the inactive carboxylate metabolite (2269W93, approximately 30% of the dose recovered in the urine). The remaining 15% of abacavir equivalents found in the urine are minor metabolites, each less than 2% of the total dose. Faecal elimination accounts for about 16% of the dose. The terminal elimination half-life of abacavir is approximately 1.5 hours. The antiviral effect of abacavir is due to its intracellular anabolite, carbovir-triphosphate (CBV-TP). When assessed by validated high-performance liquid chromatography electrospray ionization tandem mass spectrometry, CBV-TP has been shown to have a long elimination half-life (>20 hours), supporting once-daily dosing. The mean CBV-TP trough concentrations do not differ following abacavir 600-mg once-daily and 300-mg twice-daily regimens. Limited data are available for abacavir in subjects with renal dysfunction or hepatic impairment. Abacavir pharmacokinetics in HIV-infected subjects with end-stage renal disease were found to be no different from those observed in healthy adults; this finding was consistent with the kidney being a minor route of abacavir elimination. A study of abacavir pharmacokinetics in hepatically impaired adults (Child-Pugh score of 5-6) showed that the abacavir area under the plasma concentration-time curve and elimination half-life were 89% and 58% greater...

The purpose of this study was to determine the safety and pharmacokinetics of lamivudine (3TC), a nucleoside analog that has shown potent in vitro and recent in vivo activity against human immunodeficiency virus. Sixteen human immunodeficiency virus-infected patients, six with normal renal function (creatinine clearance [CLCR], > or = 60 ml/min), four with moderate renal impairment (CLCR, 10 to 40 ml/min), and six with severe renal impairment (CLCR, < 10 ml/min), were enrolled in the study. After an overnight fast, patients were administered 300 mg of 3TC orally. Blood was obtained before 3TC administration and 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 16, 24, 32, 40, and 48 h afterward. Timed urine collections were performed for patients able to produce urine. Serum and urine were assayed for 3TC by reverse-phase high-performance liquid chromatography with UV detection. Pharmacokinetic parameters were calculated by using standard noncompartmental techniques. The peak concentration of 3TC increased with decreasing renal function; geometric means were 2,524, 3,538, and 5,684 ng/ml for patients with normal renal function, moderate renal impairment, and severe renal impairment, respectively. The terminal half-life also increased with decreasing renal function; geometric means were 11.5, 14.1, and 20.7 h for patients with normal renal function, moderate renal impairment, and severe renal impairment, respectively. Both oral and renal clearances were linearly correlated with CLCR. A 300-mg dose of 3TC was well tolerated by all three patient groups. The pharmacokinetics of 3TC is profoundly affected by impaired renal function. Dosage adjustment, by either dose reduction or lengthening of the dosing interval, is warranted.